Insights into Gaseous Detectors for Ionization Measurement

Gaseous Detectors – Ionization Measurement
 
Lecture:
 
Hans-Jürgen Wollersheim
 
e-mail: h.j.wollersheim@gsi.de
Interaction of charged particles in matter
for small 
β
 
the term 
1/
β
2
 
is dominant
dE/dx has a minimum at
 
βγ
~3-4 
(minimum ionizing particle)
for high momenta dE/dx reaches  a saturation
 
= 0.3071 MeV g
-1
cm
2
liquid-H
2
He-gas
 
Bethe-Bloch formula 
describes the energy loss of heavy particles passing through matter
 
 
 
 
 
 
 
 
energy loss of a particle is
      independent of its mass!
 
 
energy loss is an important
      tool for particle identification
 
 for minimum ionizing particles 
m.i.p.
     
dE/dx ~ 1-2 MeV g
-1
 cm
2
      i.e. for a target density 
ρ
 = 1 g/cm
3
      dE/dx ~ 2 MeV/cm
Ionization detectors
Minimum-ionizing particles
 
(Sauli. IEEE+NSS 2002)
different counting gases:
 
ionization process: Poisson statistics
detection efficiency 
ε
 
depends on average number 
<n>
 
of ion pairs
Effective ionization energies
 
Large organic molecules have low-lying excited rotational states
 excitation without ionization through collisions
Charge transport in gas
Electric field   
E = 
Δ
U / 
Δ
x  
separates 
positive
 and 
negative
 charges
E
 
There is a cycle of 
acceleration 
and
 scattering/ionization 
etc.
drift(w)
 and 
diffusion (D)
 depend on field strength 
E
 and gas pressure 
ρ
 
charge diffusion
Ion mobility
For ions there is an interplay between acceleration and
collisions. Ion mobility is independent of field for a given
gas at 
ρ
, T = 
const.
E. McDaniel and E. Mason; The mobility and diffusion of ions in gases (Wiley 1973)
He
Ne
Ar
E/
ρ
 (V/cm/torr)
w
+
 ~ 10
-2
 cm/
μ
s
 
w
-
 ~ 10
1
 cm/
μ
s
ions
electrons
Electron mobility
 
drift velocities of electrons in different gases
 
drift velocities of electrons 
in Argon-Methan mixture
 
reduced electric field strength [V/cm mm Hg]
pure
 
In general, the mobility of electrons is not constant, but depend on their kinetic energy and varies with the
electric field strength.
Amplification counters
 
single-wire gas counter
 
 
gas counters may be operated in
different operation modes depending
on the applied high voltage.
Ionization chamber
 
 
An 
ionization chamber 
is operated at a
voltage which allows full collection of
charges, however below the threshold of
secondary ionization (
no amplification
).
 
For a typical field strength 500 V/cm and typical
drift velocities the collection time for 10 cm drift is
about 2 μs for e
 and 2 ms for the ions.
 
Time evolution of the signals for 
one e
-
 ion pair
:
 
planar ionization chamber
Signal collection
The motion of charges induces an
apparent current in the electrodes.
Ion causes the same signal as the
electron = same sign, same
amplitude, but much slower
Ionization chamber
no gas gain
charges move in electric field
induced signal is generated during drift of charges
induced current ends when charges reach electrodes 
 
additional ´
Frisch grid
´:
electrons drift towards Frisch grid and induce a signal but not
on the anode.
when electrons pass the Frisch grid, a signal is induced on
anode.
the 
angular dependence 
of the electrons is removed from
anode signal
252
Cf source (25k f/s)
T
1/2
=2.645 y
E
= 6.118 and 6.076 MeV
binary fission/-decay = 1/31
4
π
 
twin ionization chamber for fission fragments
measured quantities:
E
H
E
L
A
H
A
L
e
-
 drift time
segmented cathode
ϑ
φ
Fission fragment mass measurement
 
mass resolution 
σ = 3 amu
 
<108.9 amu>
 
<
143.1 amu>
 
P. Adrich, diploma thesis (2000)
Determination of the polar angles
 
P. Adrich, diploma thesis (2000)
 
drift velocity: v
drift 
= 10 (cm/
μs)
 
range of fragments in methan gas: ℓ(E,A)
 
distance cathode-anode: d = 3.8cm
Determination of the azimuthal angles
 
P. Adrich, diploma thesis (2000)
 
energy ratios for different emission angles 
ϑ
Signal generation in ionization counters
primary ionization in gases: 
I ≈ 20-30 eV/IP
 
energy loss 
Δε
:     
n
 = n
I
 = n
e
 = 
Δε
 / I     
of primary ion pairs 
n
 at x
0
, t
0
 
force:
     
F
e
 = -eU
0
/d = -F
I
 
 
energy content of capacity 
C
 
1)
 
2)
 
 
 
 
 
 
1) + 2)
 
w
+
(t)∙(t-t
0
)
 
total signal: 
electron
 &
 ion 
components
Time-dependent signal shape
 
total signal: 
electron
 &
 ion 
components
 
Both components measure 
Δε
 
and
depend on position of primary ion pair
 
x
0
 = w
-
∙(t
e
-t
0
)
 
for fast counting use only electron component!
 
drift velocities 
(
w
+
 > 0, 
w
-
 < 0)
Proportional counter
 
wire
Proportional counter
anode wire: small radius 
R
A
 ≈ 50 
μ
m 
or less
voltage
 U
0
 ≈ (300-500) V
field at 
r
 from the wire
avalanche R
I
 → R
A
 (wire radius), several mean free
paths needed pulse height mainly due to positive
ions (q
+
)
gas amplification factor (typical 10
4
–10
6
) is constant
wire
Proportional counter
 
anode wire: small radius 
r
i
 ≈ 50 
μ
m 
or less
 
voltage
 U
0
 ≈ (300-500) V
 
field at 
r
 from the wire
 
avalanche R
I
 → R
A
, several mean free paths needed
 
pulse height mainly due to positive ions (q
+
)
 
gas amplification factor (typical 10
4
–10
6
) is constant
Proportional counter
The primary produced electrons drift the anode wire and reach the area of high electrical field strength. If a
critical field strength is reached, a secondary ionization produces electrons in an avalanche.
 
primary ionization
 
secondary ionization
 
pos. ions drift due to
secondary ionization
 
time sequence of the signal evolution
Proportional counter
target
~ 
φ
lab
~ tan
ϑ
lab
entrance window
V
0
 ~ 500 V
p = 5-10 Torr
~ 3 mm 
gap anode-cathode
 
delay line
 
Δ
time 
 
Δ
t ~ tan
ϑ
M
ulti-
W
ire 
P
roportional 
C
hamber
 
Georges Charpak
Nobel price 1992
 
Cathode plate
Wires
Amplifier
 
A multi-wire proportional chamber
detects charged particles and gives
positional information on their trajectory.
 
d = 2 mm
 
charge signals
 
time resolution:           fast anode signals 
(
t
rise
 ~ 0.1 ns
)
position resolution:      for 
d = 2 mm 
σ
x
 = 50-300 
μ
m
                                                                  
(weighted with charges)
M
ulti-
W
ire 
P
roportional 
C
hamber
T
ime 
P
rojection 
C
hamber
 
Principle: 
Time Projection Chambers are based on the drift of the charge carriers with constant drift velocity 
v
D
                        in a homogenous E-field 
(E = -dU/dz).
 
typical parameters: 
E ~ 1 kV/cm, 
v
D
 ~  1-4 cm/
μ
s, 
Δ
z ~ 200 
μ
m
3-dim. 
traces: 
z
 from the drift time, (
x,y
) from the segmented anode
T
ime 
P
rojection 
C
hamber
 
for position measurement (x + y)
T
ime 
P
rojection 
C
hamber
 
for position measurement (x + y)
T
ime 
P
rojection 
C
hamber
G
as 
E
lectron 
M
ultipliers Technology
GEM-foil
 
TPC with C-pads: 100 kHz
GEM-TPCs: 10 MHz
 
238
U at 1 GeV/u
50 
μ
m Kapton, 5 
μ
m Cu on both sides, 500 V
Geiger-Müller counter
 
The discharge is not any more localized
The number of charge carriers is not any more related to the primary ionization
The gas amplification amounts to 10
8
-10
10
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Delve into the world of gaseous detectors for ionization measurement, as discussed by Hans-Jürgen Wollersheim. Topics include the Bethe-Bloch formula, ionization detectors, effective ionization energies, charge transport in gas, ion mobility, and electron mobility in gases. Explore concepts such as energy loss of particles, detection efficiency, and charge diffusion in electric fields. Gain knowledge on track ionization, minimum ionizing particles, and the interplay between acceleration and collisions in ion mobility.


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  1. Gaseous Detectors Ionization Measurement Lecture: Hans-J rgen Wollersheim e-mail: h.j.wollersheim@gsi.de Hans J rgen Wollersheim - 2020

  2. Interaction of charged particles in matter Bethe-Bloch formula describes the energy loss of heavy particles passing through matter ? ?2 2 ???2 ?2 ?2 ???? ?2 ?2 ? ?? ??= 4 ? ??2 ?? ???2 ? ? 1 2?? ?2 ? 2 ? ? ? ?,? ?2 ? = 0.3071 MeV g-1cm2 energy loss of a particle is independent of its mass! liquid-H2 energy loss is an important tool for particle identification He-gas for minimum ionizing particles m.i.p. dE/dx ~ 1-2 MeV g-1cm2 i.e. for a target density = 1 g/cm3 dE/dx ~ 2 MeV/cm for small the term 1/ 2 is dominant dE/dx has a minimum at ~3-4 (minimum ionizing particle) for high momenta dE/dx reaches a saturation Hans J rgen Wollersheim - 2020

  3. Ionization detectors incoming particle ionization track ion/e-pairs Minimum-ionizing particles (Sauli. IEEE+NSS 2002) different counting gases: ionization process: Poisson statistics detection efficiency depends on average number <n> of ion pairs ? 1 ? ? ? ? ?????? ??? ? ? Hans J rgen Wollersheim - 2020

  4. Effective ionization energies Large organic molecules have low-lying excited rotational states excitation without ionization through collisions Hans J rgen Wollersheim - 2020

  5. Charge transport in gas Electric field E = U / x separates positive and negative charges E charge diffusion in electric field ? ? ?2 4 ? ? ?? ??= ?0 ??? 4? ? ? ? drift velocity ? = 2? ? ? mean time between collisions ? = ? ? ? =? ? = ? ? ? mobility ? ? There is a cycle of acceleration and scattering/ionization etc. drift(w) and diffusion (D) depend on field strength E and gas pressure ? = ? ? = ? ? ? ? ? charge diffusion Hans J rgen Wollersheim - 2020

  6. Ion mobility ??? ???????? ?+= ?+? For ions there is an interplay between acceleration and collisions. Ion mobility is independent of field for a given gas at , T = const. ions electrons He Ne Ar E/ (V/cm/torr) w+ ~ 10-2 cm/ s w- ~ 101 cm/ s E. McDaniel and E. Mason; The mobility and diffusion of ions in gases (Wiley 1973) Hans J rgen Wollersheim - 2020

  7. Electron mobility In general, the mobility of electrons is not constant, but depend on their kinetic energy and varies with the electric field strength. drift velocities of electrons in Argon-Methan mixture drift velocities of electrons in different gases pure reduced electric field strength [V/cm mm Hg] Hans J rgen Wollersheim - 2020

  8. Amplification counters single-wire gas counter gas counters may be operated in different operation modes depending on the applied high voltage. Hans J rgen Wollersheim - 2020

  9. Ionization chamber An ionization chamber is operated at a voltage which allows full collection of charges, however below the threshold of secondary ionization (no amplification). planar ionization chamber For a typical field strength 500 V/cm and typical drift velocities the collection time for 10 cm drift is about 2 s for e and 2 ms for the ions. Time evolution of the signals for one e-ion pair: electrons ions total charge Hans J rgen Wollersheim - 2020

  10. Signal collection The motion of charges induces an apparent current in the electrodes. Ion causes the same signal as the electron = same sign, same amplitude, but much slower ? =? ?? ?? ?? ?? ?0 Hans J rgen Wollersheim - 2020

  11. Ionization chamber no gas gain charges move in electric field induced signal is generated during drift of charges induced current ends when charges reach electrodes 252Cf source (25k f/s) T1/2=2.645 y E = 6.118 and 6.076 MeV binary fission/ -decay = 1/31 additional Frisch grid : electrons drift towards Frisch grid and induce a signal but not on the anode. when electrons pass the Frisch grid, a signal is induced on anode. the angular dependence of the electrons is removed from anode signal Hans J rgen Wollersheim - 2020

  12. 4 twin ionization chamber for fission fragments measured quantities: AH AL EH EL e-drift time segmented cathode ?1?1+ ?2?2= 0 ?1?1= ?2?2 ?2 ?1= ?1+ ?2 ?1+ ?2 ?? = 103.5 0.5 ??? ?? = 108.9 0.5 ?? = 78.3 0.5 ??? ?? = 143.1 0.5 Hans J rgen Wollersheim - 2020

  13. Fission fragment mass measurement <108.9 amu> <143.1 amu> mass resolution = 3 amu P. Adrich, diploma thesis (2000) Hans J rgen Wollersheim - 2020

  14. Determination of the polar angles ??? ? =? ? ?????? angular resolution ?,? 300 4.20 drift velocity: vdrift= 10 (cm/ s) range of fragments in methan gas: (E,A) distance cathode-anode: d = 3.8cm 500 2.50 700 2.30 P. Adrich, diploma thesis (2000) Hans J rgen Wollersheim - 2020

  15. Determination of the azimuthal angles energy ratios for different emission angles E E = = 2 1 E S S V V 24 13 + + E E E 2 4 1 3 S S S S 5 . 0 V =V tan 24 5 . 0 13 P. Adrich, diploma thesis (2000) Hans J rgen Wollersheim - 2020

  16. Signal generation in ionization counters primary ionization in gases: I 20-30 eV/IP energy loss : n = nI = ne = / I of primary ion pairs n at x0, t0 force: Fe = -eU0/d = -FI energy content of capacity C ? ? =? 2?02 ?2? 1) ? ?0 ? ? 2) ? ? = ??????? ?0+ ??????? ?0 = +? ??0 ??? ??? ? w+(t) (t-t0) ? ? =? ? =? ? ? ??+? ? ? 1) + 2) ? ?0 ? ?0 total signal: electron & ion components Hans J rgen Wollersheim - 2020

  17. Time-dependent signal shape total signal: electron & ion components ? ? ??+? ? ? ? ? = ? ?0 drift velocities (w+ > 0, w- < 0) ?+? ~10 3 ? ? Both components measure and depend on position of primary ion pair x0 = w- (te-t0) for fast counting use only electron component! Hans J rgen Wollersheim - 2020

  18. Proportional counter wire Hans J rgen Wollersheim - 2020

  19. Proportional counter gas amplification factor (typical 104 106) is constant wire anode wire: small radius RA 50 m or less voltage U0 (300-500) V field at r from the wire ?0 ?? ?? 1 ? ? = ?? ? avalanche RI RA(wire radius), several mean free paths needed pulse height mainly due to positive ions (q+) Hans J rgen Wollersheim - 2020

  20. Proportional counter cloud chamber The primary produced electrons drift the anode wire and reach the area of high electrical field strength. If a critical field strength is reached, a secondary ionization produces electrons in an avalanche. time sequence of the signal evolution primary ionization secondary ionization pos. ions drift due to secondary ionization pos. ion electron Hans J rgen Wollersheim - 2020

  21. Proportional counter V0 ~ 500 V p = 5-10 Torr ~ 3 mm gap anode-cathode target ~ lab ~ tan lab entrance window t ~ tan time io ii delay line Hans J rgen Wollersheim - 2020

  22. Multi-Wire Proportional Chamber A multi-wire proportional chamber detects charged particles and gives positional information on their trajectory. Cathode plate d = 2 mm Wires Georges Charpak Nobel price 1992 charge signals Amplifier time resolution: fast anode signals (trise ~ 0.1 ns) position resolution: for d = 2 mm x = 50-300 m (weighted with charges) Hans J rgen Wollersheim - 2020

  23. Multi-Wire Proportional Chamber Hans J rgen Wollersheim - 2020

  24. Time Projection Chamber Principle: Time Projection Chambers are based on the drift of the charge carriers with constant drift velocity vD in a homogenous E-field (E = -dU/dz). typical parameters: E ~ 1 kV/cm, vD ~ 1-4 cm/ s, z ~ 200 m 3-dim. traces: z from the drift time, (x,y) from the segmented anode Hans J rgen Wollersheim - 2020

  25. Time Projection Chamber for position measurement (x + y) Hans J rgen Wollersheim - 2020

  26. Time Projection Chamber for position measurement (x + y) Hans J rgen Wollersheim - 2020

  27. Time Projection Chamber Hans J rgen Wollersheim - 2020

  28. Gas Electron Multipliers Technology GEM-foil 50 m Kapton, 5 m Cu on both sides, 500 V 238U at 1 GeV/u TPC with C-pads: 100 kHz GEM-TPCs: 10 MHz Hans J rgen Wollersheim - 2020

  29. Geiger-Mller counter The discharge is not any more localized The number of charge carriers is not any more related to the primary ionization The gas amplification amounts to 108-1010 Hans J rgen Wollersheim - 2020

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